The condensation reaction of an olefin or a mixture of olefins over a molecular sieve, or in particular a zeolite catalyst, to form higher molecular weight products is widely known. This type of condensation reaction is referred to herein as an oligomerisation reaction, and the products are low molecular weight oligomers which are formed by the condensation of up to 12, typically 2, 3 or 4, but up to 5, 6, 7, or even 8 olefin molecules with each other. As used herein, the term ‘oligomerisation’ is used to refer to a process for the formation of oligomers and/or polymers. Low molecular weight olefins (such as ethylene, propene, 2-methylpropene, 1-butene and 2-butenes, pentenes and hexenes) can be converted by oligomerisation over a molecular sieve catalyst, to a product which is comprised of oligomers and which is of value as a high-octane gasoline blending stock, which may be used or blended into a distillate type liquid fuel or as a lubricant, or as a starting material for the production of chemical intermediates and end-products. Such chemical intermediates and end-products include high purity hydrocarbon fluids or solvents, alcohols, detergents and esters such as plasticiser esters and synthetic lubricants.
Industrial oligomerisation reactions employing molecular sieve catalysts are generally performed in a plurality of tubular or chamber reactors, similar to those processes employing solid phosphoric acid (SPA) catalysts. With SPA catalysts, the pressure drop over the catalyst bed or beds is increasing gradually over the duration of the run, due to coking and/or swelling of the catalyst pellets, and the reactor run is typically terminated when a maximum allowable pressure drop over the reactor is reached. Molecular sieve catalysts do not show pressure drop increases similar to SPA catalysts. Oligomerisation reactors using molecular sieve catalysts are therefore characterised by longer reactor run lengths, and are typically decommissioned when the catalyst activity has dropped to an unacceptably low level. With these catalysts, the reactor run length that can be achieved is therefore much more sensitive to compounds or impurities in the feed that deactivate the catalyst, such as catalyst poisons. Strong bases, such as the proton bases or Bronsted bases, are known poisons for the molecular sieve oligomerisation catalysts, which are acidic. Such bases in hydrocarbon streams are often nitrogen containing compounds, such as amines and amides, and they are typically removed from feedstocks for oligomerisation reactions, including those using molecular sieve catalysts. Such organic nitrogen-containing Bronsted bases are characterised by at least one hydrogen atom bound to the nitrogen atom, and are known proton acceptors. Other organic nitrogen components do not have any hydrogen atom bound to the nitrogen, and its nitrogen atom may have three bonds to 1, 2 or 3 surrounding carbon atoms. These nitrogen atoms however still have a free electron pair, and therefore can still act as a base, termed a Lewis base. Lewis bases are known to be much weaker bases as compared to Bronsted bases, and therefore are often ignored or not considered poisons to acid catalysed processes.
U.S. Pat. No. 4,973,790 discloses a process for oligomerisation of C2 to C10 olefins over a zeolite catalyst comprising a feed pre-treatment step to remove basic nitrogen compounds. It limits itself to the removal of amines such as di-ethanol-amine, which are Bronsted bases and have the stronger basicity. It is not concerned with the nitrogen-containing Lewis bases.
U.S. Pat. No. 4,153,638 discloses a process for polymerising C2-C5 olefins to form gasoline and distillate boiling range oligomer products in the presence of a metal-substituted synthetic mica montmorillonite catalyst, which is a 2:1 layer-lattice aluminosilicate mineral, having a structure that is not capable of acting as a molecular sieve. It discloses the removal of nitrogen from the feed, but limits itself also to the removal of amines from acid gas scrubbing. U.S. Patent Application 2002/103406 A1 discloses a process for oligomerising an olefin, originating from an oxygenate to olefin process, using a nickel based catalyst. The olefin stream in the process of U.S. 2002/103406 has a low nitrogen content, as low as 0.3 ppm by weight. This stream is therefore very suitable for oligomerisation using nickel based catalysts, because these are known to be particularly sensitive to poisons, such as nitrogen compounds. The process in U.S. 2002/103406 does not comprise a treatment to remove nitrogen from the oligomerisation feed and is not using molecular sieves as oligomerisation catalysts.
U.S. Patent Application 2004/0097773 A1 discloses a process for oligomerising isobutene. It discloses the removal of nitrogen components from the feed stream, including acetonitrile and N-methyl-pyrrolidone. Both compounds are nitrogen-containing Lewis bases. The catalyst used in U.S. 2004/0097773 is a solid, acidic ion exchange resin in which some of the acidic protons have been exchanged for a metal ion.
U.S. Patent Applications 2005/0137442 A1 and 2005/0152819 A1 disclose the removal of nitrogen compounds, including a number of Lewis base compounds such as acetonitrile, N-methyl-pyrrolidone, morpholines such as N-formyl morpholine, pyridine and quinoline, from aromatic feedstocks to aromatic conversion processes using molecular sieve catalysts. These publications are not concerned with the feeds to oligomerisation processes using such catalysts, and which operate at different conditions.
We have now found that these weaker nitrogen-containing Lewis bases, when they occur in the feedstock to oligomerisation processes that employ molecular sieve catalysts, can have a surprisingly strong catalyst deactivating effect in spite of their reputation as only weak bases. Thus, there remains a need for an oligomerisation process using a molecular sieve catalyst, which process is capable of handling feedstocks comprising nitrogen-containing Lewis base components.